Debt Long-term borrowings (Details 3) (Long-term debt [Member], USD $) In Millions, unless otherwise specified	Dec. 31, 2012
MDU Resources Group, Inc [Member] \| Revolving credit agreement [Member]
Debt instrument [Line Items]
Borrowing capacity of amended revolving credit agree	$ 125.0
Ratio of funded debt to total capitalization as specified in debt convenants	65.00%
Ratio of funded debt to capitalization - Company alone, as specified in debt convenants	65.00%
MDU Resources Group, Inc [Member] \| Commercial paper revolving credit agreement [Member]
Debt instrument [Line Items]
Facility limit	125.0
MDU Energy Capital, LLC [Member] \| Master shelf agreement member [Member]
Debt instrument [Line Items]
Ratio of total debt to adjusted total capitalization as specified in debt convenants	70.00%
Ratio of subsidiary debt to subsidiary capitalization as specified in debt convenants	65.00%
Ratio of total debt to total capitalization as specified in debt convenants.	65.00%
Ratio - EBIT to interest expense	150.00%
MDU Energy Capital, LLC [Member] \| Note purchase agreemnt [Member]
Debt instrument [Line Items]
Weighted average interest rate	4.10%
Contracted issuance of senior notes	25.0
Senior notes	25.0
Centennial Energy Holdings, Inc [Member] \| Revolving credit agreement [Member]
Debt instrument [Line Items]
Ratio of total debt to total capitalization as specified in debt convenants.	65.00%
Centennial Energy Holdings, Inc [Member] \| Master shelf agreement member [Member]
Debt instrument [Line Items]
Ratio of total debt to total capitalization as specified in debt convenants.	60.00%
Ratio of EBITDA to Interest Expense	175.00%
Centennial Energy Holdings, Inc [Member] \| Note purchase agreemnt [Member]
Debt instrument [Line Items]
Issuance of senior notes	150.0
Weighted average interest rate	4.60%
Contracted issuance of senior notes	100.0
Centennial Energy Holdings, Inc [Member] \| Letter of credit [Member]
Debt instrument [Line Items]
Letters of credit	29.0
Centennial Energy Holdings, Inc [Member] \| Commercial paper revolving credit agreement [Member]
Debt instrument [Line Items]
Facility limit	$ 500.0
WBI Energy Transmission, Inc. [Member] \| Uncommitted long-term private shelf agreement [Member]
Debt instrument [Line Items]
Ratio of total debt to total capitalization as specified in debt convenants.	55.00%

Borrowing capacity of amended revolving credit agreement.

No definition available.

Name:	mdu_BorrowingCapacityOfAmendedRevolvingCreditAgree
Namespace Prefix:	mdu_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Contracted issuance of senior notes.

+ References

No definition available.

+ Details

Name:	mdu_ContractedIssuanceOfSeniorNotes
Namespace Prefix:	mdu_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Issuance of senior notes.

+ References

No definition available.

+ Details

Name:	mdu_IssuanceOfSeniorNotes
Namespace Prefix:	mdu_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Letter of credit agreement.

+ References

No definition available.

+ Details

Name:	mdu_LetterOfCreditAgreement
Namespace Prefix:	mdu_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Ratio - EBITDA to interest expense.

+ References

No definition available.

+ Details

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Namespace Prefix:	mdu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Ratio - EBIT to interest expense.

+ References

No definition available.

+ Details

Name:	mdu_RatioEbitToInterestExpense
Namespace Prefix:	mdu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Ratio of funded debt capitalization - Company alone, as specified in debt convenants.

+ References

No definition available.

+ Details

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Namespace Prefix:	mdu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Ratio of funded debt to total capitalization as specified in debt convenants.

+ References

No definition available.

+ Details

Name:	mdu_RatioOfFundedDebtToTotalCapitalizationAsSpecifiedInDebtConvenants
Namespace Prefix:	mdu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Ratio of subsidiary debt to subsidiary capitalization as specified in debt convenants.

+ References

No definition available.

+ Details

Name:	mdu_RatioOfSubsidiaryDebtToSubsidiaryCapitalizationAsSpecifiedInDebtConvenants
Namespace Prefix:	mdu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Ratio of total debt to adjusted total capitalization as specified in debt convenants.

+ References

No definition available.

+ Details

Name:	mdu_RatioOfTotalDebtToAdjustedTotalCapitalizationAsSpecifiedInDebtConvenants
Namespace Prefix:	mdu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Ratio of total debt to total capitalization as specified in debt convenants.

+ References

No definition available.

+ Details

Name:	mdu_RatioOfTotalDebtToTotalCapitalizationAsSpecifiedInDebtConvenants
Namespace Prefix:	mdu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 129

-Paragraph 2, 4

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(b),22(b))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Reflects the calculation as of the balance sheet date of the average interest rate weighted by the amount of long-term debt outstanding by type or by instrument at that time.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Including the current and noncurrent portions, carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer). Senior note holders are paid off in full before any payments are made to junior note holders.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.16)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 16

-Article 9

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.16(a))

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

Name:	us-gaap_SeniorNotes
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant